Eheim or Ultralife CO2

I'm not familiar with the Eheim system. We have the UltraLife, which is good, but you *must* replace their fine-control knob with a true needle valve. Also, you are paying for a solenoid which IMO you probably don't need. Check out Dave Gomberg's system (link on my Resouce page), but you need to add a needle valve. A diffusor needs attention and isn't 100% efficient; a reactor is low-maintenance and all the CO2 dissolves in the water.

HTH,
Jared

I'm not familiar with either CO2 setups. I put my own setups together.

I can say this though, whichever system you choose, MAKE SURE (as Jared has already said) to get a true needle valve. You need to be able to re-assuringly regulate CO2 flow at a couple bubbles a second... which as far as I know only a needle valve (aka. metering valve) can do. Needle valves also act as a checkvalve... a nice safety precaution.
Solenoids aren't needed, but are a nice addition to a CO2 system. If I could afford it, I would get one for my setup. All you do is hook up the solenoid onto the same timer as your lights. When the lights turn on, your CO2 turns on, when your lights turn off, your CO2 turns off. That in itself will double the lifespan of a CO2 tank. Assuming a 5 pond tank will last 4 months at 2 bubbles a second running 24/7, adding a solenoid will have it last 8 months. In the long run, the solenoid will pay for itself. In the money you save on refills, the gas money you save, and the time you save... and as they all say, time is money. =)

HTH
-Richer